Acid gold bath for the electroless deposition of gold

ABSTRACT

Stabilized aqueous, acid gold bath, containing a dicyanogold(I)-complex, a complex former, a reducing agent and customary additives, for electroless deposition of gold onto gold and metals that are more electronegative than gold, as well as alloys of these metals, containing a salt of hydroxylamine or a hydroxylamine derivative as reducing agent and a fluoride or hydrogen fluoride as stabilizer. Preferred embodiments include the use of an alkali- or ammonium dicyanoaurate(I) as dicyanogold(I)-complex; using a salt of hydroxylamine or a hydroxylamine derivative of the general formula ##STR1## in which R 1  and R 2  are the same or different and represent hydrogen or alkyl of 1 to 5 carbon atoms and X represents the residue of an inorganic acid, as reducing agent; using an alkali fluoride or an alkali hydrogen fluoride as stabilizer; and a pH-value less than 3, preferably from 0.5 to 2.8.

BACKGROUND OF THE INVENTION

The invention concerns a stabilized aqueous, acid gold bath containing adicyanogold(I)-complex, a complex former, a reducing agent and customaryadditives, for the electroless deposition of gold onto gold and metalswhich are more electronegative than gold, as well as alloys of thesemetals.

Gold baths for the electroless deposition of gold are already known.They involve an alkaline or acid gold bath, containing predominantly analkalidicyanoaurate(I), a complex former, a reducing agent, as well asadditives for increasing the velocity of deposition and for improvementof the adhesiveness (U.S. Pat. Nos. 4,091,128, 3,300,328, 4,154,877,3,032,436, German Offenlegungsschrifts DE-OS 2,052,787, DE-OS2,518,559). All of these baths have, as a rule, an unsatisfactorystability, decomposing during deposition of metallic gold.

These baths have the further disadvantage that at a pH-value less than 3a decomposition of the dicyanoaurate(I)-complex into difficult todissolve gold(I)-cyanide and hydrocyanic acid occurs.

The mentioned gold baths are, moreover, suited only for the gilding ofmetals that are more electronegative than gold. An optimal electrolessdeposition of gold onto gold is, on the other hand, not possible bymeans of these baths.

SUMMARY OF THE INVENTION

The present invention has the object of providing a stabilized aqueous,acid gold bath which makes possible the electroless deposition of goldonto gold and metals more electronegative than gold, as well as theiralloys.

This object will be accomplished according to the present invention witha gold bath of the above described type, thereby characterized in thatit contains a salt of hydroxylamine or a hydroxylamine derivative asreducing agent, and a fluoride or hydrogen fluoride as stabilizer.

Particular embodiments include:

that the bath contain an alkali- or ammonium-dicyanoaurate(I) asdicyanogold(I)-complex;

that the bath contain a salt of hydroxylamine or a hydroxylaminederivative of the general formula ##STR2## in which R₁ and R₂ are thesame or different and represent hydrogen or alkyl of 1 to 5 carbon atomsand X represents the residue of an inorganic acid, preferably ofhydrochloric or muriatic or sulfuric acid, as reducing agent;

that the bath contains extra alkali chloride and/or bromide as well as,if necessary, unsaturated carbonic acids; and

that the bath display a pH-value less than 3, preferably from about 0.5to 2.8.

A particular advantage of the bath according to the present invention isthat gold can be deposited without current from a stable bath onto goldsurfaces. In this manner, already present gold coatings, which are toothin, can be optionally strengthened with the aid of the bath accordingto the present invention. The bath makes possible also the gilding ofalloys, that are customary in the semiconductor industry, for example,iron-nickel- and iron-nickel-cobalt-alloys.

The bath according to the present invention has the further advantagethat the cementation of gold onto metals that are more electronegativethan gold, such as for example copper and nickel, will be promoted, andindeed through stabilization of the dicyanogold(I)-complex to pH-valuesless than 3, even at the boiling temperature of the bath.

As dicyanogold(I)-complex, all alkali-dicyanoaurate(I), for example, thesodium- and potassium-complex salts and ammoniumdicyanourate(I), aresuitable.

Expediently, the concentration can amount to between about 0.05 and 30 ggold/liter.

According to the present invention, a salt of hydroxylamine or ahydroxylamine derivative of the general formula ##STR3## is used asreducing agent, in which R₁ and R₂ are the same or different andrepresent hydrogen or alkyl of 1 to 5 carbon atoms, and X represents theresidue of an inorganic acid, preferably of hydrochloric or muriatic orsulfuric acid, and in which, as alkyl, may be mentioned for examplemethyl, ethyl, propyl, n-butyl and n-pentyl. The stability of this saltis so extraordinarily great in the acid medium of the bath according tothe present invention, that a decomposition into ammonia and dinitrogenmonoxide hardly occurs.

As stabilizer, the bath according to the present invention contains afluoride or a hydrogen fluoride, preferably an alkali fluoride or analkali hydrogen fluoride, for example, a sodium or potassium salt ofthese compounds.

To increase the velocity of deposition, it has been shown to beadvantageous to add to the bath alkali chloride and/or bromide, such as,for example, sodium chloride, potassium chloride or sodium bromide, aswell as, if necessary, unsaturated carbonic acid. Suitable carbonicacids of this type are, for example, propionic acid, aryl acid andcrotonic acid. Moreover, polyhydroxy carbonic acids, dicarbonic acidsand other complex formers, such as for example succinic acid, citricacid, nitrilotriacetic acid or ethylene diamine tetraacetic acid, canexpediently be added, since these work to accelerate the metaldeposition.

In order to adjust the pH-value to less than 3, preferably from 0.5 upto 2.8, dilute sulfuric acid will be used, which is added to the bath inrequired amounts. It is to be understood, however, that the bathaccording to the present invention is also stable at higher pH-values,and displays advantageous effectiveness.

The basic composition of the bath according to the present invention isas follows:

    ______________________________________                                        gold (as metal)    0.05-30 gram/liter                                         reducing agent     0.5-25 gram/liter                                          fluoride           1.0-30 gram/liter                                          additives          1.0-150 gram/liter.                                        ______________________________________                                    

It is of particular advantage to select a mol ratio of gold to fluoridewhich is greater than 1:1.

The operational temperature of the bath can be selected from about roomtemperature up to about boiling temperature, preferably from about 60°to 85° C.

The use of the bath according to the present invention follows in knownmanner, in that the appropriately pre-treated--depending upon thesubstrate--object is dipped in practical manner into the bath solution.

It is advantageous herewith either to stir the bath solution or toagitate the article, in order to obtain smooth, uniform surfaces.

The bath according to the present invention can be used, in particular,for the chemical gilding of metallic surfaces, such as gold, and metalsmore electronegative than gold, for example, copper, silver gold ornickel, and alloys of these metals. After suitable pre-treatment,non-metallic materials, such as, for example, made from plastic, glassor ceramic, may also be gilded.

It is of particular technical advantage that the bath according to thepresent invention works with a constant deposition velocity up to about3.0 μm/h.

A further advantage of the bath according to the present invention isthat the deposition velocity remains the same even after a standingperiod of several months.

The bath according to the present invention makes it possible to producefoils of optional thickness. The porosity of the deposit is so slightwith coating thicknesses of about 0.2 μm, that the substrate will not beattacked by 1:1 diluted nitric acid.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples of bath compositions are given by way of exampleto enable one to deposit very uniform, well adhering and ductilecoatings, under the given operational conditions.

EXAMPLE 1

    ______________________________________                                        Potassium dicyanoaurate-I                                                                            0.02 mol/liter                                         Citric acid            0.10 mol/liter                                         Potassium hydrogen difluoride                                                                        0.12 mol/liter                                         Potassium chloride     2.00 mol/liter                                         Hydroxylammoniumchloride                                                                             0.06 mol/liter                                         pH-value: 2.8                                                                 Temperature: 70° C.                                                    Deposition velocity: 0.8 μm/h.                                             ______________________________________                                    

EXAMPLE 2

    ______________________________________                                        Ammonium dicyanoaurate-I                                                                            0.015 mol/liter                                         Succinic acid         0.250 mol/liter                                         Potassium fluoride    0.120 mol/liter                                         Acrylic acid          0.125 mol/liter                                         Di-sodium salt of ethylene                                                    dinitrilotetraacetic acid                                                                           0.010 mol/liter                                         Ammonium chloride     1.200 mol/liter                                         Hydroxylammoniumsulfate                                                                             0.025 mol/liter                                         pH-value: 2.3                                                                 Temperature: 85° C.                                                    Deposition velocity: 1.2 μm/h.                                             ______________________________________                                    

EXAMPLE 3

    ______________________________________                                        Potassium dicyanoaurate-I                                                                             0.03 mol/liter                                        Citric acid             0.23 mol/liter                                        Potassium fluoride      0.15 mol/liter                                        Potassium chloride      1.50 mol/liter                                        Hydroxyldimethylammonium chloride                                                                     0.05 mol/liter                                        pH-value: 2.8                                                                 Temperature: 85° C.                                                    Deposition velocity: 0.5 μm/h.                                             ______________________________________                                    

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofmetal baths differing from the types described above.

While the invention has been illustrated and described as embodied in anacid gold bath for the electroless deposition of gold, it is notintended to be limited to the details shown, since various modificationsand structural changes may be made without departing in any way from thespirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:
 1. In a stabilized aqueous, acidicgold plating bath for the electroless deposition of gold on gold andmetals that are more electronegative than gold and alloys thereofcontaining a dicyanogold(I)-complex, a complex former and a reducingagent; the improvement comprising a stabilizer selected form the groupconsisting of an alkali metal fluoride and an alkali metal hydrogenfluoride and, as reducing agent, a hydroxyl amine derivative having theformula ##STR4## in which R₁ and R₂ are the same or different andrepresent hydrogen or alkyl of 1-5 carbon atoms and X represents theresidue of an inorganic acid.
 2. The gold bath according to claim 1,containing an alkali metal- or ammonium dicyanoaurate(I) as saiddicyanogold(I)-complex.
 3. The gold bath according to claim 1, whereinsaid inorganic acid is selected from the group consisting ofhydrochloric and sulfuric acid.
 4. The gold bath according to claim 1,further comprising an alkali metal chloride, an alkali metal bromide ormixtures thereof.
 5. The gold bath according to claim 4, furthercomprising unsaturated carbonic acids.
 6. The gold bath according toclaim 1, wherein the pH-value is less than
 3. 7. The gold bath accordingto claim 6, wherein said pH-value is between about 0.5 and 2.8.